Electroplating is the electrodeposition of a metallic coating on an electrode in order to form a surface with properties or dimensions different from those of the electrode material. The properties which may be conferred by electroplating include improved corrosion resistance, enhanced appearance, frictional characteristics, wear resistance and hardness, solderability, and specific electrical properties.
The thickness of the deposit formed by electroplating varies with the application. Where the deposit is applied for decorative purposes as little as 0.025 micrometers may be applied. Conversely, nickel-chromium deposits on automotive hardware might be as thick as 25-50 micrometers, and 1 millimeter deposits may be laid down on electroforms. Electrowinning, an electroplating technique in which the metal ions are removed in bulk amounts, may deposit as thick as two inches.
Generally, electrodeposition and electroplating have been used as surface treatment. Deposition and plating solutions are usually aqueous. Known exceptions to the use of aqueous solutions include plating of aluminum, which may be plated from organic electrolytes, and plating of tungsten, molybdenum, tantalum, aluminum, and niobium, which may be plated from fused electrolytes. The solutions usually contain additives to perform any one of several different functions such as providing a source of ions for the metal species to be deposited; providing conductivity; stabilizing the solution; buffering the pH of the solution; and aiding and modifying other properties particular to the invoIved solution. Many viable compounds accomplish more than one of the above functions.
Polymeric chelating agents, conventionally used to enhance physical characteristics of the metal being deposited, are examples of solution additive. The art is replete with patents documenting the use of polymeric additives for the purpose of brightening a plated metal species. This task was previously accomplished by post-plating methods, such as buffing, but more recently is being achieved during the plating process by addition of polymeric chelating agents. The brightening agents used are most often organic compounds and are added in small amounts, usually less than about one percent of the electroplating solution or bath.
U.S. Pat. No. 3,864,222 discloses the incorporation of polyethylene imines into gold and gold alloy plating baths as agents for the general improvement of brightness of the electroplate and of other physical properties of the deposit obtained, as well as the operating conditions of the bath.
U.S. Pat. No. 4,425,198 discloses the use of a polyacrylamide polymer as a brightener in a zinc alloy electroplating bath. Acrylamide was also used as a primary brightener for a zinc electroplating bath in U.S. Pat. No. 4,176,017.
Other brightening agents known in the art are used commonly in zinc and zinc alloy plating baths, examples of which include beta-amino-propionic acid, disclosed in U.S. Pat. Nos. 4,401,526; quarternary ammonium silicates, disclosed in 3,993,548; polypropoxy and polypropoxy-ethoxy, claimed in 3,928,149; and epihalohydrin-alkylene amine polycondensates, used in 3,869,358.
U.S. Pat. No. 4,396,647 discloses the use of a cobalt, nickel, or indium hardener as a chelate with the acid form of a methyl vinyl ether/maleic anhydride interpolymer for gold cyanide electroplating baths.
In each of the above cited patents, and in the known art, polymers of the types mentioned function as brightening agents, or agents which impart ductility, hardness, and other physical characteristics to a plated metal coating. It is noted here that in each instance, the metal plated was plated for the purpose of surface coating an existing material to enhance specific characteristics.
Electrowinning is an electrodeposition technique for extracting bulk amounts of a metal from its ore in an electrolytic cell. Hydrometallurgical processes have been used for electrowinning for the recovery of zinc, cobalt, chromium, manganese, nickel, cadmium, gallium, thalium, indium, silver, gold, and copper. The process involves subjecting the metal salt in solution to electrolysis and electrodepositing the metal at the cathode.
Direct electroplating has been used as a recovery method of sorts to remove objectionable pollutants from electroplate feed streams or solutions, as pollution is a prevalent proble with electroplating processes. In the past, the problem was solved by destruction of the objectionable metal species. The metals were precipitated as sludges and disposed of in landfills. Metal cyanides, which are among the most dangerous of chemical pollutants, are most often dealt with by destruction methods such as chlorination, electrolysis, solvent extraction and catalytic methods. Emphasis has shifted, however, to the direct recovery of objectionable metals for reuse in the plant or for resale to refiners. Proposed recovery methods for metals less dangerous than cyanides include reverse osmosis, evaporative recovery, ion exchange, and combinations thereof. High cost of equipment, among other costs particular to specific methods, such as membrane replacement in reverse osmosis, make such methods prohibitive in many instances.
A specific instance where the recovery of desirable metal species may be important is in affinity dialysis and other similar soluble affinity adsorbent systems. The conventional means for removal of metal species from the feed solution of such systems is to bind the metal species to an affinity adsorbent in an extraction step and then strip the metal from the soluble adsorbent at a later stage of the process. This requires use of a strip reagent, which destroys the binding complex and releases the metal species to be stripped. The strip reagent comprises an aqueous solution of an acid or base, depending on the charge character of the material to be stripped. Where the system is a continuous flow system, this addition of reagent may then further necessitate regeneration, or adjustment of the pH, of the affinity adsorbent by further addition of acid or base solution prior to its recirculation through the system. Such processes require costly chemicals, extensive pumping equipment, and highly sensitive controls for monitoring of system parameters such as temperature, pH, and feed flow to affinity adsorbent flow ratio.
What is lacking in the art is a means for efficient recovery or removal of metals in substantially pure bulk form from feed solutions containing a polymeric chelating agent which has adsorbed the metal species. Also lacking is a bulk metal ion recovery or removal process applicable in either single pass or continuous flow systems.
Therefore, it is an object of the present invention to provide a feasible method for the recovery of desirable metals as well as the removal of objectionable metals from solutions.
It is also an object of the present invention to provide a method for the recovery of desired metal species in a substantially pure form from a polymeric chelating solution.
It is a further object of the present invention to provide a method for the recovery of metal species from affinity adsorbent systems in an efficient and simple manner.
These and additional objects of the present invention will become apparent in the description of the invention and examples that follow.